Heteroaromatic compounds Heterocyclic rings

The present review deals with condensed systems of heteroaromatic five-membered rings—thienothiophenes, di enothiophenes, seleno-phenoselenophenes, and selenophenothiophenes. The number of publications devoted to such compounds has grown rapidly in recent years. This is indicative of the mounting interest in various aspects of this field of heterocyclic chemistry, which, however, has not been reviewed since the 1954 monograph of Hartough and Meisel. ... 

Prototropic Tautomerism of Heteroaromatic Compounds III, Five-Member Rings and One Hetero Atom A. R. Katritzky and J. M. Lagowski, Adv. Heterocycl. Chem., 1963, 2, 1-81. 

The increase in energy in a molecule on absorption of UV light is sufficient to bring about bond cleavage. As a result, fragmentation and rearrangement of the molecule can occur. The effect on heterocycles is discussed in this section and, for simplicity, the transformations are classified, somewhat arbitrarily, on the basis of ring size pyrazolines are treated separately. Heterocyclic dienes and heteroaromatic compounds are also discussed separately, and the section is completed by consideration of the photochemistry of heteroaromatic A-oxides. 

The coupling of aryl- and alkenylzinc reagents with various halides has widespread use in the cross-coupling of aromatic rings [101]. The reactions of zinc derivatives of aromatic and heteroaromatic compounds with aryl and heterocyclic halides have wide synthetic applications [103,104],... 

Heterocyclic compounds (like carbocyclic compounds) can be divided into heteroaromatic and heteroalicyclic types. In general, the chemistry of heteroalicyclic compounds is similar to that of their aliphatic analogues, but that of heteroaromatic compounds involves additional principles. Aromatic compounds possess rings in which (1) each of the ring atoms is in the same plane and has a p orbital perpendicular to the ring plane and (2) (4n + 2) Jt-electrons in cyclic conjugation are associated with each ring. 

Pyridinium(imidazolium) inner azolate salts and molecules with a betaine character of general type 1 are attractive substrates from the viewpoint of structural chemistry, as mentioned in the Introduction. This ensemble of compounds offers the possibility of two terminal heterocyclic rings, joined through several spacers, with extreme characteristics within heteroaromatic systems a 7r-deficient nucleus (cation) and a 7r-excessive nucleus (anion). The high dipolar character is the distinctive feature offered by these compounds and has a powerful influence on their physical and chemical properties. 

The eighth volume of this serial publication comprises eight contributions from international authors. Four of these deal with well-defined groups of compounds thiopyrones (R. Zahradnik, R. Mayer, and W. Broy), phenoxazines and phenothiazines (M. Ionescu and H. Mantsch), diazepines (F. D. Popp and A. Catala Noble), and benz-isoxazoles (anthranils and indoxazenes) (K.-H. Wiinsch and A. J. Boulton). J. M. Tedder has surveyed the field of the heteroaromatic diazo compounds which are derived from a variety of heterocyclic ring systems, and M. Schulz and K. Kirschke discuss heterocyclic peroxides. The remaining two chapters survey well-known reactions the Hilbert-Johnson reaction is covered by J. Pliml and M. PrystaS, and heterocyclic Claisen rearrangements by B. S. Thyagarajan. 

Heterocyclic compounds are cyclic structures which contain an atom other than carbon as part of a ring system. Heteroaromatic compounds are a group of these which possess a cyclic conjugated system that can satisfy the (4 + 2)n electron rule and thus have some aromatic stabilization. 

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